A Three-Dimensional Study of the Influence of Mountains on a Front

Abstract

The objective of this study is to investigate mountain effects on a frontal system in three dimensions. The frontal system is developed from the most unstable Eady wave in a baroclinic state without a mountain. The developed frontal system is then introduced into a new model domain that contains mountains with different sizes, shapes, and orientations. In general, it is found that the cold front experiences a weakening on the upwind slope and strengthening on the downwind slope of a mountain. The locations of these upwind and downwind sides are determined by the horizontal winds associated with the front. Before the front reaches a mountain, the prevailing wind impinging on the mountain is the prefrontal southwesterly. After the front reaches the top of the mountain, the impinging wind shifts to be the postfrontal northwesterly. Therefore, mountain-induced fronto-genetic forcing by these winds varies spatially as the front passes the mountain. When the front moves down the slope, it speeds up and the frontal deformation is then caused by the strong advection over the northern part of the mountain. After the front has moved away from the mountain, its original horizontal structure and location are restored. The frontogenetic forcing is dominated mainly by the convergence-divergence associated with the flow over the mountain. The front experiences major intensification when it is in the leeside convergence zone. As the front moves farther downstream, it enters the divergence zone and its intensity is reduced. When the front has moved away from the influence of the mountain, its intensity returns approximately to its original level irrespective of the mountain's size and shape. The postfrontal winds contribute to the strong convergence, which causes enhanced lee frontogenesis. For an east-west oriented elliptic mountain that resembles the Alps, the Ieeside downslope wind induced by the postfrontal flow is toward the south instead of toward the east as in the other cases. Therefore, the front moves with an average speed that is the same as the front with no mountain. In this case, the front also has a net increase in its intensity for the same period of integration. Simulations with this mountain profile compare favorably with many observed phenomena near the Alps. Overall, the most important factor that determines the net effect of the mountain on a front is its orientation relative, to the front.